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Life Cycle of a Star (Cloud of dust and gas/Nebulae (Protostar (Main…
Life Cycle of a Star
Cloud of dust and gas/Nebulae
Protostar
Main sequence
Red giant
The helium core continues to contract until it gets hot enough and dense enough for helium to fuse
A huge amount of energy is released and the outer layers of the star get pushed further outwards
Once all the helium in the core is fused, the star starts to collapse again under its own weight
The core will contract again and heat up but it will not reach a high enough temperature to continue to fuse any of the elements still in the core
Once the core has shrunk to about Earth-size, electrons will exert enough pressure to stop it collapsing any more - electron degeneracy
As the core contracts, the outer layers become more and more unstable
The star pulsates and ejects its outer layers into space as planetary nebula
White dwarf
All the fusion reactions have stopped
A white dwarf slowly cools down while it uses up the energy in its core
Black dwarf
The white dwarf will eventually change colour and become a black dwarf
Red supergiant
A red supergiant has much more fuel than a red giant but it gets used up more quickly
Some stars can continue fusing elements until their core is made of iron - nuclear fusion does not continue beyond iron
If the mass of the core is more than 1.4 times the mass of the Sun, the electrons get squashed onto the atomic nuclei, combining with protons to form neutrons and neutrinos
The core suddenly collapses to become a neutron star, which the outer layers fall onto
When the outer layers hit the surface of the neutron star they rebound, setting up huge shockwaves, ripping the star apart and causing a supernova
Supernova + Neutron star
Neutron stars are incredibly dense, very small and rotate very quickly
If the mass of the core is more than 3 times the mass of the Sun, the neutrons can't withstand the gravitational forces and the star collapses to form a black hole
Black hole
A black hole is an infinitely dense point in space
The gravitational pull is so strong that even light is attracted
Stars spend most of their lives as main sequence stars
The pressure produced from hydrogen fusion in a star's core balances the gravitational force trying to compress it - hydrostatic equilibrium
When all the hydrogen in the core has been fused to helium, nuclear fusion stops
This means the outward pressure from the nuclear fusion is no longer present
The gravitational force causes the helium core to contract and heat up further
The outer layers expand and cool
Dense clump of dust and gas that continues to contract and heat up
When the temperature reaches a few million degrees, the hydrogen nuclei start to fuse together to form helium
The fusion releases an enormous amount of energy and creates enough radiation pressure to stop the gravitational collapse
Protostars with masses less than 0.08 solar masses and greater than 0.0125 solar masses are called brown dwarfs
Brown dwarf
Brown dwarfs never reach temperatures high enough for the fusion of hydrogen to begin
Brown dwarfs can fuse deuterium during their life
Sub-brown dwarf
If a sub-brown dwarf orbits around another stellar object, it is called a planet
There is no nuclear fusion in a sub-brown dwarf
Protostars with masses less than 0.0125 solar masses are called sub-brown dwarfs
Left when previous stars blew themselves apart in supernovae
The denser clumps of the cloud contract under the force of gravity
When the clumps get dense enough, the cloud fragments
In each of the fragments, the collapsing gas releases gravitational potential energy as heat
As the temperature and pressure increase, a fragment condenses into a rotating sphere of superhot gas called a protostar